GaN-based high electron mobility transistors (HEMTs) have a great prospect in the field of large-current, low-power-consumption, high-frequency and high-voltage switching applications due to high electron saturation speed, high two-dimensional electron gas (2DEG) density and high critical breakdown electric field.
The key of power switching devices is to achieve high breakdown voltage, low power consumption, and high reliability. The critical breakdown electric field of GaN material is ten times that of Si. The breakdown voltage of GaN power device is far lower than its theoretical limit at present. One of the important reasons is that the electric field crowding at the gate leads to premature breakdown. At this time, the drift region is not completely depleted.
The field plate technology is a common terminal technology to improve the breakdown voltage of the device. A Reference (J. Li, et.al. “High breakdown voltage GaN HFET with field plate” IEEE Electron Lett., Vol. 37, no. 3, pp. 196-197, February 2001.) discloses a field plate which is shortly connected to the gate. As shown in FIG. 1. The introduction of field plate can reduce the curvature effect of the main junction and electric field peaks, thereby increase the breakdown voltage. However, the introduction of the field plate increases the parasitic capacitance of the device, which adversely affects the high frequency and switching properties of the device. Reference (Akira Nakajima., et.al. “GaN-Based Super Heterojunction Field Effect Transistors Using the Polarization Junction Concept” IEEE Electron Device Letters, vol. 32, no. 4, pp. 542-544,2011) uses the concept of polarization super junction. A top layer of GaN is gown on the AlGaN barrier layer of the drift region, and two-dimensional hole gas (2DHG) is formed at the interface thereof 2DHG and the underlying 2DEG form a natural “super junction” to assist the depletion of the drift region when blocking the breakdown voltage of the device, so as to optimize the horizontal electrical field, and thus the purpose of improving the breakdown voltage is achieved, as shown in FIG. 2. However, the top layer of GaN and the gate form Ohm contact of holes. Thus, when the device is forward conductive, if the gate voltage is large, leaking current will occur at the gate, which limits the gate voltage swing.
For AlGaN/GaN devices, it is well-known that enhancement mode HEMT devices have more advantages than depletion mode HEMTs. Therefore, researchers have a great interest in how to realize the enhancement mode. Reference (W. Saito, et.al., “Recessed-gate structure approach toward normally off high-voltage AlGaN/GaN HEMT for power electronics applications,” IEEE Trans. Electron Devices, vol. 53, no. 2, pp. 356-362, February 2006) adopts the trench gate structure to achieve a quasi-enhancement mode AlGaN/GaN HEMT, as shown in FIG. 3. Recessed gate etching can effectively deplete the 2DEG concentration under the gate, so the threshold voltage is greatly increased. However, recessed gate etching requires precise control of the etch depth and also may cause etch damage. Common ways to achieve the enhancement mode also include fluoride ions treatment in barrier layer under the gate. P-type GaN gate, etc. These methods all realize the enhancement anode by depleting 2DEG under the gate, which will inevitably lead to the contradiction between high threshold voltage and large saturation output current.